rust: add `alloc` crate

rust: add `alloc` crate

This crate is a subset of the Rust standard library `alloc`, with some
additions on top.

This is needed because upstream support for fallible allocations
is a work in progress (i.e. the `try_*` versions of methods which
return a `Result` instead of panicking).

Having the library in-tree also gives us a bit more freedom to
experiment with new interfaces and allows us to iterate quickly.

Eventually, the goal is to have everything the kernel needs in
upstream `alloc` and drop it from the kernel tree.

Co-developed-by: Alex Gaynor <alex.gaynor@gmail.com>
Signed-off-by: Alex Gaynor <alex.gaynor@gmail.com>
Co-developed-by: Wedson Almeida Filho <wedsonaf@google.com>
Signed-off-by: Wedson Almeida Filho <wedsonaf@google.com>
Co-developed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Gary Guo <gary@garyguo.net>
Co-developed-by: Matthew Bakhtiari <dev@mtbk.me>
Signed-off-by: Matthew Bakhtiari <dev@mtbk.me>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>